Chapter 11 Production of Scotch and Irish whiskies: their history and ...

Production of Scotch and Irish whiskies: their history and evolution 163Figure 13. Gas liquid chromatograms of aromacompounds produced by yeast in a nitrogen-free sugarfermentation, with a trace for comparison of the aromacompounds detected in a sample of Scotch whisky(Suomalainen and Nykänen, 1966).The complexity of the processes for determiningthe presence and concentration of compoundsin whisky may be illustrated by looking at onecompound for which this process has beenelucidated in detail. In the 1980s ethyl carbamate,a naturally-occurring component of manyalcoholic drinks, was identified as beingundesirable. Canada specified by regulation amaximum limit for ethyl carbamate in whisky spiritof 150 ppb and the US set guidelines of 120ppb. Control of this compound was only possibleafter the processes resulting in its presence inspirit were fully understood. Extensive researchin Scotland revealed the mechanism of ethylcarbamate production and facilitated theintroduction of very effective control measures.These measures maintain levels close to zeroand always less than 30 ppb in distilled whiskyspirit.Cook (1990) reviewed the outcome of thisresearch and the resulting control procedures.When barley sprouts during malting, a glycoside,epiheterodendrin (EPH) is present in theacrospire. This glycoside, which survives kilningand mashing, is extracted into the wort andconverted by yeast enzymes to glucose andisobutyraldehyde cyanohydrin (IBAC). The IBACis stable during fermentation, but when heatedabove 50 o C at distillation it breaks down to formvolatile nitriles. There are a multiplicity of potentialroutes nitriles can follow including reaction withother beer components or complex reactionsoften mediated indirectly by copper. Some ofthe volatile nitriles may escape these reactionsand pass into the distillate. A number of reactionscan remove the nitriles from the spirit, one beingthe reaction with ethanol to form ethylcarbamate.Two control strategies may be used toprevent ethyl carbamate reaching the final spirit.Firstly, some varieties of barley produce low levelsof the glycoside EPH (Cook, 1990), and plantbreeders are now concentrating onincorporating this character in all varieties for thedistilling industry. The second strategy relies onthe low volatility of ethyl carbamate. Control ofdistillation conditions eliminates all the volatileprecursor formed by copper mediated reactions.Non-volatile substances are formed prior to thefinal distillation in malt distillation or beforerectification in grain distillation. Thus, ethylcarbamate can be minimized in the final spirit.A considerable number of studies have beenpublished on those organoleptically-importantcompounds arising either directly or indirectlyfrom the oak barrels in which whisky is matured.The increase in coloring, tannin, dissolved solidsand acid concentrations are not observed whenwhisky is stored in glass, which is proof of theimportance of the oak barrels in the maturationprocess. An analysis of heartwood of theAmerican oak (Quercus albus) gave cellulose(49-52%), lignin (31-33%), pentosans (orhemicelluloses, 22%) and compounds extracted